WELDING PROCESS AND FEATURES OF MICROSTRUCTURE OF WELDED JOINTS OF BIOPOLYMERS

Polymers and biopolymers in particular are increasingly used in various sectors of the economy and more recently biopolymers replace traditional polymers in many applications. The problem of polymeric materials recycling can be solved by designing products that will be easy to recycle. In fact, biodegradation is the consistent breaking of chemical links of a polymer molecular chain under the action of microorganisms. Destroying a polymer, bacteria, fungi or algae use the remnants of its molecules as a source of vital organic compounds as well as energy. Usually biodegradation occurs in an aqueous or humid environment during the process of composting. Bioplastics wastes, like fallen leaves or other organic waste, are stacked on soil and gradually converted into environmentally friendly material. The ability of a polymer to biodegrade mainly depends on the chemical composition of its macromolecule chain. It must be heterocyclic and should include biodegradable chemical groups. The chain should not include fragments consisting of more than five CH2 groups in sequence. The breakdown of the molecular chain is facilitated by the presence in its structure of bulk substituents of hydrogen atoms with a sufficiently large volume. It is also possible to include in the chain of natural organic groups such as starch, cellulose, urea and the like. In this work, the samples were formed by pressing of the polyhydroxybutyrate (PHB) powder under the following temperature conditions: the powder heated up to 40°C was poured into the mold and heated without pressure for 1-2 min. After that it was kept under pressure for 12-15 min., and then it was heated up to 175°C for 3-5 min. According to the experimental data, it was found that with increasing of the pressing temperature internal stresses appeared in the specimens, and defects such as cracks occurred upon cooling. Under lower pressing temperatures, the powder was not completely melted and according to, the samples had a non-homogeneous microstructure. Therefore when welding the temperature of the heater was set at 220-240°C, the upset pressure - 0.1 MPa. The warm-up time was varied within 30-40 seconds to determine the optimum conditions of weld formation. It was confirmed that PHB material is a brittle material with high fluidity when melted. In the welded joints, flash rollers are usually formed with the melt spreading over the sample surface. The flash has a small height almost equal on the both sides of the welded joint.